Swans are
birds of the
family Anatidae within the genus
Cygnus. The swans' close relatives include the
geese and
ducks. Swans are grouped with the closely related geese in the
subfamily Anserinae where they form the
tribe Cygnini. Sometimes, they are considered a distinct subfamily,
Cygninae. There are six or seven species of swan in the genus
Cygnus; in addition there is another species known as the
coscoroba swan, although this species is no longer considered one of the true swans. Swans usually
mate for life, though "divorce" does sometimes occur, particularly following nesting failure, and if a mate dies, the remaining swan will take up with another. The number of
eggs in each ranges from three to eight.